1. Field of Invention
This invention relates to gate valves, particularly to a gate valve of the type employing a camming wedge which shifts valve elements into positions which seal the valve's openings.
2. Description of Prior Art
Gate valves are widely used to control fluid flow through pipes in industrial, domestic, and commerical installations, pipelines, or the like.
One such valve is described in U.S. Pat. No. 2,502,689 to C. W. Yant, April 1950. This prior-art valve is shown schematically in FIG. 1 of the present drawings. It consists of a valve housing 10 which contains a first plate 12, a second plate 14, and a movable wedge 16 between the plates. Plates 12 and 14 have tapering surfaces 18 and 20 which are in constant contact with the respective sides of wedge 16 under the effect of compression springs 22 and 24. Housing 10 is rectangular in shape and has a first side 26 and a second side 28 with through openings 30 and 32, respectively. Sides 26 and 28 have flanges (not shown) mounted thereon so that the valve can be installed in a pipe (not shown).
The wide end of wedge 16 is connected to a drive rod 35 of a drive unit 34 which may comprise, e.g., a pneumatic cylinder (not shown). An O-ring 29 is mounted in the outer surface of plate 14. When the valve is closed, this ring aids in sealing plate 14 against side 28 over opening 32.
In operation, drive unit 34 moves wedge 16 and plates 12 and 14 toward end stop 36. Until the ends of plates 12 and 14 contact the surface of end stop 36, the plates move with wedge 36. When the ends of plates 12 and 14 contact the inner side of stop 36, the plates must stop, but drive rod 35 will continue to push wedge 16 further to the left, to the position shown in FIG. 1. This action will cam plates 12 and 14 radially outwardly until O-ring 29 and plate 14 seal opening 32. Plate 12 will also come in contact with side 26, but as there is no O-ring to seal the space between plate 12 and side 26, atmospheric pressure will always be present inside the housing.
The prior-art valve, although simple in construction and inexpensive to manufacture, had a serious operational problem: When one desired to open the valve in the presence of a considerable pressure differential across the valve, he or she would cause drive rod 35 to move to the right so as to open the valve. Wedge 16 is pulled to the right forming a gap with plates 12 and 14. The force of springs 22 and 24 will shift plate 12 down. Atmospheric pressure inside the housing, however, will keep second plate 14 in place. Thus drive unit 34 must overcome the friction created by this force. As a result, drive unit 34 must be made more powerful than if such friction were not present.
In addition, when the drive unit pulls wedge 16 further to the right, friction will be present between plate 14 and side 28, causing plate 14 and its O-ring 29 to drag against side 28. This will wear O-ring 29 more rapidly than if such pressure were absent. As a result, the O-ring requires frequent replacement. Such replacement involves labor and material costs, plus additional expenses associated with shutting off the system or the pipeline.
Thus the industry would benefit by an improved gate valve free of the above-described disadvantages.